Why does a PDB require an upgrade?

Why do I need to upgrade (or downgrade) a Pluggable Database (PDB) once I unplug it from version A and replug into version B? This question is one of the most often asked questions in our workshops when Roy and I present Upgrades and Oracle Multitenant. If you take a look into the documentation you’ll find: But unfortunately this is not 100% correct for every object. When you do a simple query for basic dictionary structures such as OBJ$ you’ll easily find out that a PDB has its own Data Dictionary.  Some things are really linked into the PDB only –…